CN115642629A - Bipolar flexible direct current system and method for eliminating alternating current bias component - Google Patents

Abstract

The invention discloses a bipolar flexible-direct system and an elimination method of alternating current offset components, when a single-phase-to-ground fault or a bridge arm flashover fault at a valve side of a transformer is detected, the problem that alternating current at a network side of the transformer does not have zero point is solved by grounding a direct current side of a positive electrode of a fault converter station and closing a quick isolating switch of the fault converter station; when the alternating current-direct current line touch ungrounded fault is detected between the direct current line and the adjacent alternating current line, the problem that alternating current of the same-side non-fault pole converter station and the alternating current line does not have zero crossing is solved through grounding of the positive direct current side of the fault converter station and the negative direct current side of the same-side non-fault pole converter station and closing of a quick isolating switch between the two-stage lines of the fault converter station and the same-side non-fault pole converter station; and when detecting that the line-touching ground fault of the AC/DC line occurs between the DC line and the adjacent AC line, tripping three phases of the parallel AC line and the AC circuit breaker of the non-fault pole converter station, and executing a circuit breaker tripping instruction when the current of the AC side of the fault converter station crosses zero.

Description

A Bipolar Flexible Straight System and Elimination Method of AC Current Bias Component

technical field

The invention relates to the technical field of flexible direct current transmission, in particular to a bipolar flexible direct current system and a method for eliminating an alternating current bias component.

Background technique

Flexible DC transmission technology (Voltage Source Converter High Voltage DirectCurrent, VSC-HVDC) has the advantages of flexible control, no commutation failure, and the ability to supply power to passive systems. It can realize multi-terminal DC transmission or DC networking. Key technologies for energy Internet construction. Flexible DC transmission technology based on Modular Multi-level Converter (MMC) has become the solution to regional grid interconnection, wind power and other new energy grid integration due to its unique technical advantages such as low harmonic content and easy modularization. Preferred option.

At present, most of the flexible DC projects that have been put into operation in the world are symmetrical unipolar systems based on Half Bridge Sub-module (HBSM). The soft straight system will be widely developed and applied. Compared with the symmetrical unipolar structure, the bipolar flexible direct current transmission system has higher reliability, larger transmission capacity, and flexible and changeable operation mode, which can realize the independent operation of positive and negative poles. However, compared with the symmetrical unipolar flexible DC transmission system, the symmetrical bipolar structure system has some technical characteristics, such as DC voltage bias on the valve side of the converter transformer, which puts forward certain requirements for the insulation design of transformers and other equipment. In addition, under typical fault conditions such as single phase-to-ground fault on the transformer valve side and bridge arm flashover fault, due to the structural characteristics of the half-bridge sub-module, a DC bias component will be generated in the AC current of the transformer grid side or valve side of the converter station. , thus affecting the normal tripping of the transformer network side or valve side AC circuit breaker, threatening the safe and reliable operation of the system.

At present, the AC and DC systems of the domestic AC power grid are in mixed operation, and the phenomenon of crossover of high-voltage DC lines and high-voltage AC lines is relatively common. With the increasing tension in the transmission corridor, there may be scenarios where AC and DC lines are erected on the same tower, so it needs to be considered How to improve the safety and reliability of the system operation under the two fault conditions of the AC and DC line touching the line and not grounding fault, and the AC and DC line touching the line and grounding fault.

Contents of the invention

Therefore, the technical problem to be solved by the present invention is to overcome the defects in the prior art that the AC current lacks a zero-crossing point and the AC circuit breaker cannot normally trip under fault conditions, thereby providing a bipolar flexible straight system and an AC current bias component method of elimination.

To achieve the above object, the present invention provides the following technical solutions:

In the first aspect, an embodiment of the present invention provides a bipolar flexible straightening system, including: a positive flexible straightening system and a negative flexible straightening system. The return line is connected, and one end of the metal return line is directly grounded; both the positive flexible straight system and the negative flexible straight system include a sending-end converter station, a receiving-end converter station, a sending-end converter transformer, a receiving-end converter transformer, and a sending-end AC circuit breaker. converter, AC circuit breaker at the receiving end, and multiple fast grounding switches; the converter station at the sending end is connected to the AC busbar at the sending end in turn through the converter transformer at the sending end, the AC circuit breaker at the sending end; The converter transformer and the receiving-end AC circuit breaker are connected to the receiving-end AC busbar; the DC side of the sending-end converter station is connected to the DC side of the receiving-end converter station through a DC overhead line; the DC overhead line and the parallel AC line are erected on the same tower; The two ends of the overhead line are respectively grounded through the quick grounding switch.

In an embodiment, both the positive pole flexible DC system and the negative pole flexible DC system further include: a plurality of fast disconnectors; the DC sides of the sending-end converter station and the receiving-end converter station are both connected in parallel to the fast disconnectors.

In an embodiment, the bipolar flexible straight system further includes: parallel AC lines and AC circuit breakers; two ends of the parallel AC lines are respectively connected to the sending-end AC busbar and the receiving-end AC busbar through an AC circuit breaker.

In one embodiment, the bipolar flexible DC system further includes: a plurality of starting resistors; the sending-end converter transformer is connected to the sending-end AC circuit breaker through the starting resistors; the receiving-end converter transformer is connected to the receiving-end AC circuit breaker through the starting resistors .

In one embodiment, the converter transformer at the sending end and the converter transformer at the receiving end adopt a Δ/Y or Y/Δ connection mode.

In the second aspect, the embodiment of the present invention provides a method for eliminating the bias component of the AC current. The method is used to eliminate the bias component of the AC current in the bipolar flexible DC system and its adjacent AC system in the first aspect. The method includes: judging the bipolar Whether the DC system is faulty or not, when it is faulty, determine the type of fault; based on the type of fault, through the control, fast isolating switch, AC circuit breaker on the grid side of the transformer, and the on-off status of the AC circuit breaker on the parallel AC line, and Isolate faulty converter stations.

In one embodiment, the fault types include: transformer valve side single phase-to-ground fault, bridge arm flashover fault, AC-DC line touch-line non-ground fault, and AC-DC line touch-line ground fault.

In an embodiment, when the fault type is a single phase-to-ground fault on the transformer valve side or a bridge arm flashover fault, the process of isolating the faulty converter station includes: closing the fast grounding switch of the positive DC outlet line of the faulty converter station, or Close the fast isolating switch connected in parallel on the DC side of the faulty converter station; when the AC side current on the transformer grid side of the faulty converter station crosses zero, control the AC circuit breaker on the transformer grid side of the faulty converter station to trip.

In one embodiment, when the fault type is an AC/DC line touch-line non-ground fault, the process of isolating the faulty converter station includes: closing the fast grounding switch of the positive DC outlet line of the faulty converter station, switching non-faulty poles on the same side The fast grounding switch of the negative DC outlet line of the converter station, or close the fast isolating switch connected in parallel on the DC side of the faulty converter station, or the fast isolating switch connected in parallel on the DC side of the non-faulty pole converter station on the same side; When the current on the AC side of the pole converter station and the parallel AC line current cross zero, the AC circuit breaker that controls the non-faulty pole converter station on the same side and the parallel AC line is tripped.

In one embodiment, when the fault type is a line-to-ground fault of an AC/DC line, the process of isolating the faulty converter station includes: controlling the AC circuit breaker of the parallel AC line, and tripping the AC circuit breaker of the non-faulty pole converter station on the same side; When the current on the AC side of the faulty converter station crosses zero, the AC circuit breaker of the faulty converter station is controlled to trip.

The technical solution of the present invention has the following advantages:

The bipolar flexible DC system and the method for eliminating the bias component of the AC current provided by the present invention, when a single-phase-to-ground fault on the transformer valve side or a bridge arm flashover is detected, the positive DC side of the faulty converter station is urgently grounded and the faulty converter is closed. The fast isolating switch between the two-level lines of the converter station can solve the problem of no zero-crossing point of the AC current on the transformer grid side of the faulty converter station; The emergency grounding of the negative DC side of the non-faulty pole converter station on the side, and the fast isolating switch between the two-stage lines of the closed faulty converter station and the non-faulty pole converter station on the same side can solve the problem of the non-faulty pole converter station on the same side and the AC line. There is no zero-crossing problem for AC current; when a grounding fault is detected on the AC-DC line, firstly make the parallel AC line and the AC circuit breaker of the non-faulty pole converter station perform three-phase tripping. , and then execute the circuit breaker trip command.

Description of drawings

In order to more clearly illustrate the specific implementation of the present invention or the technical solutions in the prior art, the following will briefly introduce the accompanying drawings that need to be used in the specific implementation or description of the prior art. Obviously, the accompanying drawings in the following description The drawings show some implementations of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative work.

Fig. 1 is a composition diagram of a specific example of a bipolar flexible straightening system in an embodiment of the present invention;

Fig. 2 is a topological diagram of a converter station in an embodiment of the present invention;

Fig. 3 is a composition diagram of another specific example of the bipolar flexible straightening system in the embodiment of the present invention;

FIG. 4 is a flowchart of a specific example of a method for eliminating an AC current bias component in an embodiment of the present invention;

Fig. 5 is the timing sequence of the protection cooperation action under the fault of the AC and DC lines erected on the same tower in the embodiment of the present invention when the line touches the line and is not grounded.

Detailed ways

The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Apparently, the described embodiments are some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" etc. The indicated orientation or positional relationship is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the referred device or element must have a specific orientation, or in a specific orientation. construction and operation, therefore, should not be construed as limiting the invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and should not be construed as indicating or implying relative importance.

In the description of the present invention, it should be noted that unless otherwise specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection or a detachable connection. Connected, or integrally connected; it can be mechanically or electrically connected; it can be directly connected, or indirectly connected through an intermediary, or it can be the internal communication of two components, which can be wireless or wired connect. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention in specific situations.

In addition, the technical features involved in the different embodiments of the present invention described below may be combined with each other as long as there is no conflict with each other.

Example 1

An embodiment of the present invention provides a bipolar straightening system, including: a positive flexible straightening system and a negative flexible straightening system.

Specifically, the positive flexible straight system and the negative flexible straight system are directly grounded at the DC neutral point or connected through a metal return line, and one end of the metal return line is directly grounded;

Further, both the positive flexible DC system and the negative flexible DC system include a sending-end converter station, a receiving-end converter station, a sending-end converter transformer, a receiving-end converter transformer, a sending-end AC circuit breaker, a receiving-end AC circuit breaker, Multiple quick earth switches.

Furthermore, the sending-end converter station is connected to the sending-end AC bus through the sending-end converter transformer and the sending-end AC circuit breaker in sequence; the receiving-end converter station is connected to the receiving-end converter transformer and receiving-end AC circuit breaker in sequence The DC side of the sending end converter station is connected to the DC side of the receiving end converter station through the DC overhead line; the DC overhead line and the parallel AC line are erected on the same tower; the two ends of the DC overhead line are respectively passed through the fast grounding switch grounded. The converter transformer at the sending end and the converter transformer at the receiving end adopt the Δ/Y or Y/Δ connection mode.

Exemplarily, taking the topology shown in Figure 1 as an example, in Figure 1, HVDC1, HVDC2, HVDC3, and HVDC4 are all converter stations, T1, T2, T3, and T4 are all converter transformers, and S1, S2, S3, S4 is a fast grounding switch, QF1, QF2, QF3, and QF4 are AC circuit breakers, QF1, T1, HVDC1, S1, S2, HVDC2, T2, and QF2 form a positive flexible straight system, and QF3, T3, HVDC3, S3, and S4 , HVDC4, T4, QF4 constitute the negative flexible straight system. Wherein, the converter station may adopt a modular multilevel converter topology based on the half-bridge sub-module structure as shown in FIG. 2 , which is not limited here.

In a specific embodiment, both the positive pole flexible straight system and the negative pole flexible straight system further include: a plurality of fast disconnectors; the DC sides of the sending-end converter station and the receiving-end converter station are both connected in parallel to the fast disconnectors.

Exemplarily, as shown in FIG. 3 , HSS1 , HSS2 , HSS3 , and HSS4 are fast isolation switches for HVDC1 , HVDC2 , HVDC3 , and HVDC4 , respectively.

In a specific embodiment, as shown in Figure 3, the bipolar flexible straight system also includes: parallel AC lines and AC circuit breakers (QF5, QF6 in Figure 3); the two ends of the parallel AC lines pass through an AC circuit breaker respectively , correspondingly connected with the AC busbar at the sending end and the AC busbar at the receiving end. Among them, the AC lines running in parallel share the AC busbar with the DC transmission system, and the AC lines and the DC lines are erected on the same tower.

In a specific embodiment, as shown in Figure 3, the bipolar flexible DC system further includes: a plurality of starting resistors (R1, R2, R3, R4 in Figure 3); The AC circuit breaker is connected; the converter transformer at the receiving end is connected to the AC circuit breaker at the receiving end through a starting resistor.

Example 2

An embodiment of the present invention provides a method for eliminating the bias component of an AC current, which is applied to the bipolar flexible straight system in Embodiment 1, as shown in FIG. 4 , the method includes:

Step S11: Determine whether the bipolar flexible straight system is faulty, and when it is faulty, determine the type of fault.

Specifically, the embodiment of the present invention judges the type of fault based on changes in the electrical information characteristics of the bipolar DC system when various types of faults occur. Fault at F1), bridge arm flashover fault (fault at F2 in Fig. F4 in 3 fails).

Among them, when a single-phase-to-ground fault on the transformer valve side occurs due to insulation breakdown or bushing flashover, the corresponding protection will be triggered and cause the converter valve to block. , the fault current will form a current path fed into the fault point through the lower bridge arm. At this time, a voltage difference will be generated on the AC and DC sides of the converter station, resulting in a DC bias component of the short-circuit current on the AC side, which will further cause a short circuit of the faulty phase on the valve side of the transformer. The current has no zero-crossing point, and the two-phase short-circuit current on the grid side has no zero-crossing point, which affects the normal tripping of the AC circuit breaker on the transformer grid side or valve side.

Bridge arm flashover is a serious fault type that affects the normal operation of the converter valve. When the converter valve is blocked, the fault current fed from the AC side will form a path through the healthy phase and the faulty bridge arm. The existence of a fault will generate a bias component in the AC side current of the converter, resulting in no zero-crossing point for the short-circuit current of the faulty phase on the valve side of the transformer, and no zero-crossing point for the two-phase short-circuit current on the grid side, affecting the AC circuit breaker on the grid side or valve side of the transformer. normal trip.

When there is a non-ground fault between the AC single-phase line and the DC unipolar line (that is, the non-ground fault of the AC and DC line), the fundamental frequency component will be introduced into the DC line current, and at the same time, due to the line coupling effect, it will be in the AC A DC bias component develops in the line current. In addition, the fault current of the AC line will be fed into the neutral point grounding point of the transformer grid side of the DC system through the connection point of the AC busbar. Since the AC and DC sides of the converter station where the faulty pole is located have the same potential, the AC circuit breaker current of the faulty station will be There will be no bias component, but since the potential of the DC side of the non-faulty converter station is higher than that of the AC side, a DC bias component will be generated in the AC current of the non-faulty pole converter station. Similarly, the fault current of the DC line will be fed into the AC line through the connection point of the AC bus bar, resulting in a bias component of the faulty phase short-circuit current on the AC line, and finally the AC circuit breaker current on the transformer network side cannot trip due to the lack of zero-crossing points. As a result, faults cannot be isolated in time, which brings risks to the safe and stable operation of the system.

When a line-to-earth fault occurs between the AC single-phase line and the DC unipolar line (that is, the line-to-ground fault of the AC-DC line), the fundamental frequency component will be introduced into the DC line current, and at the same time due to the line coupling effect in the AC line current Generates a short-duration DC bias component. After the converter valve is locked, since the fault current is mainly fed from the neutral ground point to the fault ground point through the diode of the faulty pole converter station, the potential of the DC side of the faulty pole converter station is higher than that of the AC side, and there will be The DC bias component will be generated in the three-phase current of the transformer network side, and the fault phase current bias decays slowly. On the other hand, for the non-faulty pole converter station, there is no fault current path in the converter station, and the transformer grid side current will have a short-term DC bias, but the value is small and decays quickly, which does not affect the normal tripping of the circuit breaker. For parallel AC lines, a bias component with a small value and fast decay will also be generated in the fault phase current.

Step S12: Based on the fault type, the faulty converter station is isolated by controlling the on-off status of the fast grounding switch, the fast disconnecting switch, the AC circuit breaker on the transformer grid side, and the AC circuit breaker on the parallel AC line.

Specifically, single-phase-to-ground faults on the valve side of transformers and bridge arm flashover faults will cause the current on the AC side of the faulty converter station to be zero; line touch line without grounding fault) will cause current bias, that is, the AC side current of the non-faulty pole converter station on the same side of the faulty converter station and the parallel running AC line current lack a zero-crossing point; The DC line of the flexible DC system and the parallel running AC line touch the ground fault) will cause the current bias problem, that is, the current on the AC side of the faulty converter station lacks a zero-crossing point.

In view of the above problems, the embodiments of the present invention implement different operation mechanisms for different topological structures and different fault types, so as to solve the problem of no zero-crossing point of AC current. For example: the emergency grounding of the DC side of the converter station, the connection between the two-stage lines of the converter station through a quick disconnect switch, and the sequential tripping strategy of the AC circuit breaker.

In a specific embodiment, when the fault type is a single phase-to-ground fault on the transformer valve side or a bridge arm flashover fault, the process of isolating the faulty converter station includes:

Step 1: Close the fast grounding switch of the positive DC outlet line of the faulty converter station, or close the fast disconnecting switch connected in parallel on the DC side of the faulty converter station.

Step 2: When the AC side current on the transformer grid side of the faulty converter station crosses zero, the AC circuit breaker on the transformer grid side of the faulty converter station is controlled to trip.

Exemplarily, taking the single-phase-to-ground fault on the transformer valve side at F1 in Figure 3 as an example, HVDC1 is the faulty converter station, first close S1, and then control QF1 to trip when the current on the AC side of HVDC1 crosses zero; or, first Close HSS1, and then control QF1 to trip when the HVDC1 AC side current crosses zero.

Exemplarily, taking the bridge arm flashover fault at F2 in Figure 3 as an example, HVDC1 is the faulty converter station, first close S1, and then control QF1 to trip when the HVDC1 AC side current crosses zero; or, first close HSS1 , and then control QF1 to trip when the current on the AC side of HVDC1 crosses zero.

In a specific embodiment, for AC/DC line touch line non-grounding faults, considering the difference in the influence of different line fault locations on the converter station, the converter station that blocks the converter valve is defined as the faulty converter station. At this time, the faulty converter station The DC side of the converter station and the non-faulty pole converter station on the same side need to be grounded urgently, as shown in the fault F3 near the DC side of the HVDC1 station in Figure 3. The procedure for a faulty converter station includes:

Step 3: Close the fast grounding switch of the positive DC outlet line of the faulty converter station, the fast grounding switch of the negative DC outlet line of the non-faulty pole converter station on the same side, or close the fast isolation switch connected in parallel on the DC side of the faulty converter station 1. Fast disconnectors connected in parallel on the DC side of the non-faulty pole converter station on the same side.

Step 4: When the current on the AC side of the non-faulty pole converter station on the same side and the parallel AC line current cross zero, control the tripping of the AC circuit breaker of the non-faulty pole converter station on the same side and the parallel AC line.

Illustratively, taking the AC-DC line touch line non-ground fault at F3 in Figure 3 as an example, HVDC1 is the faulty converter station, and HVDC3 is the non-faulty pole converter station on the same side, first close S1 and S3, and then when HVDC3 AC When the side current and the parallel AC line current cross zero, control QF3 and QF5 to trip; or first close HSS1 and HSS3, and then control QF3 and QF5 to trip when the HVDC3 AC side current and parallel AC line current cross zero.

In steps 1 to 4, the step of controlling the fast grounding switch and the AC circuit breaker is characterized in that the fast grounding switch is installed at the DC side outlet of the positive pole line of the faulty pole converter station and the DC side outlet of the negative pole line of the non-faulty pole converter station, which can Realize rapid closing and have a certain current resistance capacity. Both the grid-side circuit breaker and the AC line circuit breaker of the converter station of the flexible DC system are three-phase integrated circuit breakers. This solution can be used to solve the current and Parallel operation of the AC line current has no zero-crossing point, and the AC circuit breaker cannot trip, which can effectively reduce the overcurrent stress of the converter valve.

In steps 1 to 4, the characteristic of the step of controlling the fast disconnecting switch and the AC circuit breaker is that the fast disconnecting switch (HSS) is installed between the positive and negative lines of the DC side of the converter station, which can realize fast closing and has a certain resistance The grid side circuit breaker is a three-phase integrated circuit breaker. This solution can be used to solve the problem that the AC side current of the converter station and the parallel operation AC line current have no zero crossing point and the AC circuit breaker cannot trip under typical faults. Converter valve overcurrent stress.

In a specific embodiment, when the fault type is an AC/DC line touch-to-ground fault, the process of isolating the faulty converter station includes:

Step 5: Control the tripping of the AC circuit breaker of the parallel AC line and the AC circuit breaker of the non-faulty pole converter station on the same side.

Step 6: When the current on the AC side of the faulty converter station crosses zero, the AC circuit breaker of the faulty converter station is controlled to trip.

As an example, take the AC-DC line touch-to-ground fault at F4 near the DC side of HVDC1 station in Figure 3 as an example. HCC1 is the faulty converter station, and HVDC3 is the non-faulty pole converter station on the same side. , QF3 tripping, when the HVDC1 AC side current crosses zero, control QF1 tripping.

For AC and DC line touch-to-ground faults, the implemented steps (step 5 and step 6) do not require additional equipment, and the sequential tripping strategy of the AC circuit breaker and AC line circuit breaker in the converter station is used to solve the AC-DC line touch-to-ground fault. Under fault conditions, the current of the AC circuit breaker of the faulty pole converter station has no zero-crossing point and cannot trip, which can effectively reduce the overcurrent stress of the converter valve.

For the steps implemented in steps 1 to 6, taking the failure of F1, F2, and F3 in Figure 3 as an example, the fast earthing switch or fast disconnecting switch that needs to be closed is shown in Table 1.

Table 1

Further, the protection configuration of the AC/DC parallel operation system shown in FIG. 3 of the embodiment of the present invention for AC/DC line touch-line non-ground faults is as follows:

AC system: distance protection is the main protection, and AC differential protection is the backup protection.

DC system: Valve overcurrent protection and AC/DC touch line protection are the main protections, and DC line overcurrent protection is the backup protection.

For AC and DC line touch-line non-ground faults, the AC-DC system protection action sequence considering the coordination of line touch protection is shown in Figure 5.

When a fault occurs, for the DC system, the overcurrent protection of the valve will be triggered in 1~2ms to cause the blockage of the converter station, and the line touch protection will be triggered about 40~60ms after the fault, and the grounding switch of the DC side or the pole of the converter station will be completed after 35~70ms. The closing operation of the fast isolating switch between the lines, Δt1 is the decay time of the current bias component, generally 1-2 cycles, after the current bias on the AC side of the non-faulty converter station disappears, the station’s AC is completed in about 41-60ms circuit breaker tripping operation;

For the AC line, the fault will trigger the protection action. After the closing operation of the grounding switch on the DC side or the fast isolating switch between the pole lines of the converter station is completed, the bias component of the Δt2 current will gradually decay to 0, and then the AC circuit breaker will trip. If the reclosing fails, it will be judged as a permanent fault and trip three phases.

Apparently, the above-mentioned embodiments are only examples for clear description, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. However, the obvious changes or changes derived therefrom are still within the scope of protection of the present invention.

Claims (10)

1. A bipolar straightening system, comprising: a positive electrode flexible and straight system and a negative electrode flexible and straight system, wherein,

the positive flexible direct system and the negative flexible direct system are directly grounded by a direct current neutral point or are connected through a metal return wire, and one end of the metal return wire is directly grounded;

the positive flexible-direct system and the negative flexible-direct system respectively comprise a transmitting end converter station, a receiving end converter station, a transmitting end converter transformer, a receiving end converter transformer, a transmitting end alternating current breaker, a receiving end alternating current breaker and a plurality of quick grounding switches;

the transmission end converter stations are connected with the transmission end alternating current bus through transmission end converter transformers and transmission end alternating current circuit breakers in sequence; the receiving end converter stations are connected with a receiving end alternating current bus through a receiving end converter transformer and a receiving end alternating current breaker in sequence; the direct current side of the transmitting end converter station is connected with the direct current side of the receiving end converter station through a direct current overhead line; the direct current overhead line and the parallel alternating current line are erected on the same tower;

and two ends of the direct current overhead line are respectively grounded through the quick grounding switch.

2. The bipolar straightening system according to claim 1, wherein the positive straightening system and the negative straightening system each further comprise:

a plurality of fast disconnect switches;

and the direct current sides of the sending end converter station and the receiving end converter station are connected with the quick isolating switch in parallel.

3. The bipolar straightening system according to claim 2, further comprising:

parallel AC lines and AC breakers;

and two ends of the parallel alternating current circuit are correspondingly connected with the sending end alternating current bus and the receiving end alternating current bus through an alternating current breaker respectively.

4. The bipolar straightening system according to claim 1, further comprising:

a plurality of starting resistors;

the transmission end converter transformer is connected with the transmission end alternating current circuit breaker through the starting resistor;

and the receiving end converter transformer is connected with the receiving end alternating current circuit breaker through the starting resistor.

5. The bipolar straightening system according to claim 1,

the transmitting end converter transformer and the receiving end converter transformer adopt a delta/Y or Y/delta connection mode.

6. A method for eliminating an ac bias component in a bipolar flexible direct current system as claimed in any one of claims 3 to 5 and adjacent ac systems, the method comprising:

judging whether the bipolar flexible direct system has a fault or not, and judging the type of the fault when the bipolar flexible direct system has the fault;

and based on the fault type, the fault converter station is isolated by controlling the on-off states of a quick grounding switch, a quick isolating switch, an alternating current breaker on the transformer network side and an alternating current breaker of a parallel alternating current line.

7. The method of eliminating an alternating current bias component of claim 6, wherein the fault type comprises:

the fault of the transformer valve side is single relative to the ground, the fault of the bridge arm flashover, the fault of the AC/DC line touching without the ground and the fault of the AC/DC line touching with the ground.

8. The method for eliminating the alternating current offset component according to claim 7, wherein when the fault type is a transformer valve side single phase to ground fault or a bridge arm flashover fault, the process of isolating the fault converter station comprises:

closing a quick grounding switch of a positive electrode direct current outlet circuit of the fault converter station or closing a quick isolating switch connected in parallel at the direct current side of the fault converter station;

and when the current at the alternating current side of the transformer network side of the fault converter station crosses zero, controlling the alternating current breaker at the transformer network side of the fault converter station to trip.

9. The method according to claim 7, wherein when the fault type is an ac/dc line-to-line ungrounded fault, the isolating the faulty converter station comprises:

closing a quick grounding switch of a positive electrode direct current outlet circuit of the fault converter station, a quick grounding switch of a negative electrode direct current outlet circuit of the non-fault electrode converter station on the same side, or closing a quick isolating switch connected in parallel at the direct current side of the fault converter station and a quick isolating switch connected in parallel at the direct current side of the non-fault electrode converter station on the same side;

and when the current at the alternating current side of the non-fault pole convertor station at the same side and the current of the parallel alternating current line have zero crossing points, controlling the alternating current circuit breakers of the non-fault pole convertor station at the same side and the parallel alternating current line to trip.

10. The method according to claim 7, wherein when the fault type is an ac/dc line-to-line ground fault, the isolating the faulty converter station comprises:

controlling the AC circuit breakers of the parallel AC lines and the AC circuit breakers of the same-side non-fault pole converter station to trip;

and when the current at the alternating current side of the fault converter station has a zero crossing point, controlling an alternating current breaker of the fault converter station to trip.

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